Fuel cells are devices that directly convert chemical energy of reactants, i.e., fuel and oxidant, into direct current (DC) electricity. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel and more efficient than portable power storage, such as lithium-ion batteries.
In general, fuel cell technologies include a variety of different fuel cells, such as alkali fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and enzyme fuel cells. Today's more important fuel cells can be divided into three general categories, namely fuel cells utilizing compressed hydrogen (H2) as fuel, proton exchange membrane (PEM) fuel cells that use methanol (CH3OH), sodium borohydride (NaBH4), hydrocarbons (such as butane) or other fuels reformed into hydrogen fuel, and PEM fuel cells that use methanol (CH3OH) fuel directly (“direct methanol fuel cells” or DMFC). Compressed hydrogen is generally kept under high pressure, and is therefore difficult to handle. Furthermore, large storage tanks are typically required, and cannot be made sufficiently small for consumer electronic devices. Conventional reformat fuel cells require reformers and other vaporization and auxiliary systems to convert fuels to hydrogen to react with oxidant in the fuel cell. Recent advances make reformer or reformat fuel cells promising for consumer electronic devices. DMFC, where methanol is reacted directly with oxidant in the fuel cell, is the simplest and potentially smallest fuel cell, and also has promising power application for consumer electronic devices.
DMFC for relatively larger applications typically comprises a fan or compressor to supply an oxidant, typically air or oxygen, to the cathode electrode, a pump to supply a water/methanol mixture to the anode electrode and a membrane electrode assembly (MEA). The MEA typically includes a cathode, a PEM and an anode. During operation, the water/methanol liquid fuel mixture is supplied directly to the anode, and the oxidant is supplied to the cathode. The chemical-electrical reaction at each electrode and the overall reaction for a direct methanol fuel cell are described as follows:
Reaction at the Anode:CH3OH+H2O→CO2+6H++6e−
Reaction at the Cathode:O2+4H++4e−→2H2O
The Overall Fuel Cell Reaction:CH3OH+1.5O2→CO2+2H2O
Due to the migration of the hydrogen ions (H+) through the PEM from the anode through the cathode and due to the inability of the free electrons (e−) to pass through the PEM, the electrons must flow through an external circuit, which produces an electrical current through the external circuit. The external circuit may be any useful consumer electronic devices, such as mobile or cell phones, calculators, personal digital assistants and laptop computers, among others. DMFC is discussed in U.S. Pat. Nos. 5,992,008 and 5,945,231, which are incorporated by reference in their entireties. Generally, the PEM is made from a polymer, such as Nafion® available from DuPont, which is a perfluorinated material having a thickness in the range of about 0.05 mm to about 0.50 mm, or other suitable membranes. The anode is typically made from a Teflonized carbon paper support with a thin layer of catalyst, such as platinum-ruthenium, deposited thereon. The cathode is typically a gas diffusion electrode in which platinum particles are bonded to one side of the membrane.
The cell reaction for a sodium borohydride reformer fuel cell is as follows:NaBH4(aqueous)+H2O→(heat or catalyst)→(H2)+(NaBO2)(aqueous)H2→(2H++2e−)+O2→2H2O(at the anode)2(2H++2e−)+O2→2H2O(at the cathode)Suitable catalysts include platinum and ruthenium, among other metals. The hydrogen fuel produced from reforming sodium borohydride is reacted in the fuel cell with an oxidant, such as O2, to create electricity (or a flow of electrons) and water byproduct. Sodium borate (NaBO2) byproduct is also produced by the reforming process. Sodium borohydride fuel cell is discussed in United States published patent application no. 2003/0082427, which is incorporated herein by reference.
The patent literature discloses a number of non-pressurized and pressurized portable fuel tank or fuel storage for fuel cells. United States patent application publication no. 2002/0018925 A1 discloses an electronic device with a cavity, where a refillable balloon containing fuel is stored. This balloon is made from an elastic material. United States patent application publication no. 2003/0008193 A1 discloses a flexible walled fuel tank that contains fuel and an absorbent material.
U.S. Pat. No. 6,460,733 B2 discloses a multi-walled fuel tank comprising an inner fuel container disposed inside an outer container. The inner container may have a rigid, semi-rigid or flexible wall. The plenum area between the two containers comprises agents or additives that neutralize methanol fuel in case of breakage or before disposal. The fuel is fed to a fuel reservoir or directly to the anode electrode by gravity or by a pressurized gas source located within the outer container. An external pump is also provided to communicate the fuel to the fuel cell.
U.S. Pat. No. 6,506,513 B1 discloses, among other things, a fuel tank comprising a pressure adjusting mechanism for maintaining a constant pressure within the tank and an inner bellow containing fuel. United States patent publication nos. 2002/0197522 and 2003/0082427 disclose a fuel cartridge comprising a fuel bladder and a pressurized mechanism applied to the fuel bladder. Publication '427 further discloses a bladder adapted to receive liquid byproduct(s) from the fuel cell.
United States patent application publication no. US 2002/0127451 A1 discloses a compact PEM fuel cell that stores methanol fuel directly in contact with the PEM, and the CO2 byproduct is vented to the housing to pressurize the housing. This housing further comprises a relief valve to prevent the over-pressurization of the housing, and a fuel intake valve to add fuel. This reference does not disclose a separate fuel tank or cartridge.
Recently, Neah Power Systems of Bothell, Wash., disclosed a sealed fuel tank that stores fuel as well as water and CO2 byproducts in separate bladders.
However, the prior art does not disclose a fuel cartridge with a flexible inner chamber that minimizes residual fuel trapped in the inner chamber when the walls of the chamber come in contact with each other, or fuel cartridges with the advantages and features described below.